The invention relates to an electronic photometer with a photoelectric transducer in an evaluating circuit, at the output of which circuit there is connected at least one of two luminescent indicators, the brightnesses of which indicators can be matched for determination of the scene brightness, by setting an adjustment device to vary the value of a parameter of illumination.
Light meters of this kind generally present two lamps, one of which has a constant brightness, thereby delivering a reference brightness. The other lamp on the contrary is variable in its luminous power, being supplied from the output of the evaluating circuit so that upon adjustment of a parameter of illumination by the grey wedge or shutter connected in series with the photoelectric transducer, or by changing the resistance within the evaluating circuit, the brightness of said second lamp is varied. What is involved here is a subjective light measurement, because the eye of the observer has to determine the match of the two brightnesses. Obviously methods of photometry of this kind are weighted with error. However, they afford the user a greater freedom in selection of his parameters of illumination than a fully automatic control would allow.
The present invention is addressed to the problem of producing a photometer of the mentioned type, in which the advantages of known light meters are retained, but which allows greater accuracy in spite of subjective assessment of the brightness. This is effected in terms of the invention in that there is a first combining device (14; 47, 48, 49, 55) and a second combining device connected in a series with respective indicators (54) to each combining device the output signal (U2) of the evaluating circuit (2-12; 21-39) can be applied on the one hand, and on the other hand a reference signal (U4; 56), and in that one combining device (14; 55) is arranged to combine both signals by means of a computing operation, and the other combining device (16, 17; 40, 41, 55) is arranged to combine by an inverse computing operation, whereby the brightnesses of the two indicators are inversely changeable. The essence of the invention resides therefore in the fact that instead of a reference lamp with constant brightness, both indicators are variable, presenting the same brightness when correctly set. Hereby also the difference between the two brightnesses is substantially greater, and the adjustment point at which the brightnesses are equal is more readily determined. The combination can be effected by both addition and subtraction, as well as by multiplication and division etc. Preferably, each indicator presents a luminescent diode because there is then the additional advantage that the diode acts as a light emitter and also functions as a rectifier that blocks when there is too little or too much illumination, so that the diode is extinguished.
Another means of increasing the accuracy may be such that there is associated with the evaluating circuit a device controlling at least one indicator, for delinearizing the light characteristic of said indicator, to enhance the indicating sensitivity in the brightness match range of the two indicators. This measure can also be used independently of the light meters that have been mentioned, i.e., also with a photometer in which an indicator is lit at constant brightness. Precisely in the range in which the brightness difference is most poorly judged subjectively, the sensitivity is increased.